58 SNAKE RIVER PLAINS OF IDAHO. [bull. 199. 
the depressions. There is evidence tending to show that lacustral con- 
ditions continued after the highest sheet of lava in the region below the 
mouth of Bruneau River was poured out, but these higher beds, being- 
unprotected by a hard cover, have been to a great extent eroded away. 
The volcanic eruptions were accompanied by the blowing out of large 
quantities of volcanic dust and lapilli; other fragmental deposits were 
produced when the lava flows, which, as is presumed, followed the 
explosive eruptions, entered water bodies. 
ECONOMIC PROBLEMS. 
As to the economic importance of the sedimentary beds just con- 
sidered, I may say briefly that possibly some of the clays may be found 
suitable for brickmaking, should commercial conditions make this 
desirable. It has been suggested that the abundant outcrops of fine 
clay near Hagerman can be utilized in the manufacture of aluminum, 
the waterfalls near at hand furnishing the necessary power for electric 
smelting. The feasibility of such a plan, however, remains to be 
ascertained. 
The principal interest, from an economic point of view, at present 
attached to the sedimentary beds referred to is in reference to the 
possibility of obtaining artesian water. As has been stated, these 
beds are in part close-textured clays and in part loose sands and 
gravel. The beds of lapilli are in general compact and do not allow 
the free percolation of water through them. One of the requisite 
conditions for the storage of subterranean water under pressure is 
thus present; that is, an alternation of pervious and impervious strata. 
Another requisite is that the succession of water-bearing and non- 
water-bearing beds should be inclined, or, best of all, so bent as to 
form a basin. In the portion of the Snake River Plains to the west 
of Salmon Falls the strata are not horizontal, but inclined gently 
downward from the bases of the bordering mountains toward the 
center of the plains; that is, the central part of the basin has been 
depressed, or its border raised, in reference to a nearly east- west axis, 
so as to give the strata a trough-like form. The longer axis of this 
trough below Salmon Falls has a gentle downward inclination or pitch 
to the northwest. This trough has the structural features of what is 
known as an artesian basin, but in the region thus far explored is only 
a portion of a completely inclosed basin, the location of its western 
margin being as yet unknown. Whether or not the major axis of 
the trough rises again to the west can not be told at present, but 
from such general knowledge as is in hand concerning the western 
portion of the Snake River Plains and the occurrence of pre-Tertiary 
rocks in the canyon of Snake River below Weiser, it is presumed that 
